1. Field of the Invention
The present invention relates in general to a hybrid drive system for a motor vehicle, and more particularly to techniques for reducing a shock generated upon changing of an operation mode of the hybrid drive system, upon shifting of an automatic transmission from one forward drive position to another forward drive position, upon changing of a ratio of torque distribution to front and rear wheels of the vehicle, or upon shifting of the automatic transmission from a non-drive position such as a neutral position to a drive position such as a forward or reverse drive position.
2. Discussion of the Related Art
JP-A-7-67208 discloses an example of a known hybrid drive system of a motor vehicle, which is equipped with a drive power source including an engine operated by combustion of a fuel, and an electric motor operated with an electric energy. At least one of the engine and the electric motor is operated in a selected one of a plurality of operations modes, for driving the motor vehicle.
For example, the operation modes of the hybrid drive system includes an engine drive mode in which only the engine is operated as the drive power source to drive the vehicle, a motor drive mode in which only the electric motor is operated as the drive power source, and an engine.cndot.motor drive mode in which both the engine and the electric motor are operated as the drive power source. One of the operation modes is selected on the basis of suitable operating parameters of the vehicle, for instance, the vehicle running speed (or the speed of the currently selected drive power source) and the operating amount of an accelerator pedal, and according to a suitable mode selecting or changing pattern as represented by a stored data map.
Such a hybrid drive system may include a power transmitting device in the form of an automatic transmission which is disposed between the drive power source and drive wheels of the vehicle and whose speed ratio is variable. The speed ratio of the automatic transmission is selected or changed on the basis of suitable operating parameters of the vehicle, for instance, the vehicle running speed and the operating amount of the accelerator pedal, and according to a suitable transmission shifting pattern as represented by a stored data map. Between the drive power source and the drive wheels of the vehicle, a torque distributing mechanism may also be disposed for changing the ratio of torque distribution to the front and rear drive wheels. The torque distribution ratio is selected or changed on the basis of suitable parameters such as the yaw rate, steering angle and running speed of the vehicle, and according to a suitable torque distribution changing pattern as represented by a stored data map.
Where the hybrid drive system is adapted such that the control to select or change the operation mode, the control to shift the automatic transmission and the control to select or change the torque distribution ratio are effected independently of each other, and operation to select the operation mode may take place concurrently with a shifting action of the automatic transmission or an operation to change the torque distribution ratio. In this case, the hybrid drive system tends to have a considerable variation in the vehicle drive torque, and difficulty to suitably control the drive power source, automatic transmission and torque distribution mechanism, and is likely to suffer from a shock upon simultaneous operations to change the operation mode, shift the automatic transmission and/or change the torque distribution ratio. The shifting actions of the automatic transmission may include a so-called "clutch-to-clutch shifting action wherein one of two frictional coupling devices is engaged while the other frictional coupling device is released, to shift the transmission from one gear position to another. In such a clutch-to-clutch shifting action, the pressure of the frictional coupling device that is to be engaged is required to be intricately controlled according to the input torque of the transmission. If the clutch-to-clutch shifting action is effected while at the same time the operation mode is changed, a torque variation due to the operation to change the operation mode undesirably deteriorates the accuracy of control of the engaging pressure of the above-indicated frictional coupling device, leading to easy generation of a shifting shock of the automatic transmission.
The shifting pattern of the automatic transmission may be formulated so as to maximize the energy efficiency or minimize the fuel consumption of the vehicle, for each of the operation modes. In this case, shift boundary lines for one operation mode are not continuous with shift boundary lines for another operation mode. This may cause frequent shifting actions of the automatic transmission upon changing of the operation mode, resulting in a change in the speed ratio of the automatic transmission in a relatively short time. FIG. 11A shows examples of mode changing patterns and transmission shifting patterns. Described more specifically, hatched area in FIG. 11A indicates a motor drive condition in which the motor drive mode is selected, while non-hatched area indicates an engine drive condition in which the engine drive mode is selected. As is apparent from FIG. 11A, the engine drive mode is selected when the vehicle running speed V or the engine load as represented by the operating amount .theta..sub.AC of the accelerator pedal is comparatively high. In the motor drive mode, the automatic transmission is shifted according to shift boundary lines (transmission shifting pattern) indicated by broken lines. In the engine drive mode, the automatic transmission is shifted according to shift boundary lines indicated by solid lines. In FIG. 11A, "1", "2", "3" and "4" indicate first-, second-, third- and fourth-speed positions of the automatic transmission. As indicated in the figure, the shift boundary lines for the motor drive mode are not continuous with the shift boundary lines for the engine drive mode, at a boundary between the motor drive condition and the engine drive condition. In the arrangement of FIG. 11A, therefore, a change of the vehicle operating or running state from point A to point B, for example, will cause a change of the operation mode from the motor drive mode to the engine drive mode, and a simultaneous shift-up action of the automatic transmission from the second-speed position to the third-speed position. A change of the vehicle running state from point A' to point B' will cause a change of the operation mode from the motor drive mode to the engine drive mode, and a simultaneous shift-up action of the automatic transmission from the third-speed position to the fourth-speed position, which is followed by a shift-down action from the fourth-speed position to the third-speed position.
The automatic transmission, which is a power transmitting device adapted to transmit power from the drive power source to the drive wheels of the vehicle, may have a plurality of drive positions having respective different speed ratios, which are selectively established by selective engagement and disengagement of coupling devices such as clutches and brakes. Alternatively, the automatic transmission may be a continuously variable transmission of belt-and-pulley type or toroidal type whose speed ratio is continuously variable. The automatic transmission as the power transmitting device has non-drive positions such as a neutral position "N" and a parking position "P", and drive positions such as a drive position "D" and a reverse position "R". These non-drive and drive positions are selectively established by manually operated transmission switching means such as a combination of a shift lever and a manual shift valve mechanically connected to the shift lever.
In such a hybrid drive system having a power transmitting device and manually operated transmission switching means, the power transmitting device may not be smoothly shifted from a non-drive position to a drive position, or may suffer from a shifting shock, due to a change in the output of the drive power source, which may arise from a change in the operating amount of the accelerator pedal or a change in the operation mode, when the power transmitting device is commanded to be shifted from the non-drive position to the drive position.